This invention relates to fault current limiters.
Fault currents are large (usually temporary) increases in the normal current flowing in a power transmission system. A fault current can occur from any number of different events including lightning strikes or catastrophic failure of electrical equipment which can cause short circuits. A short circuit, for example, can cause a twenty-fold or more increase in current flow to the circuit.
Conventional circuit breakers are used in virtually every power transmission and distribution system to "open" the circuit and interrupt current flow in the event of a fault. The fault current level grows as new equipment is added over time. However, with an increase in the magnitude of fault current comes an increase in the size and expense of the circuit breaker. Moreover, conventional circuit breakers do not open instantaneously. The fault current is generally first detected by a current sensor which generates a signal to a control circuit. The control circuit processes the signal and then generates a control signal to open the circuit breaker. During these steps (which may have a duration as long as 100 msec), the circuit breaker, as well as other parts of the transmission system are subjected to the higher fault current level. Thus, the circuit breaker, transformers, as well as other components of the system are often rated to withstand, for a period of time,the higher current levels.
Fault current limiters were developed to insert impedance in a connection quickly so as limit the magnitude of the fault current to a reduced level, thereby protecting the circuit breaker and the power transmission system. Many fault current limiters include tuned reactance circuits which store energy in proportion to the circuit inductance.
The transition characteristics of superconducting materials have been used advantageously to develop superconducting fault current limiters. For example, in one conventional approach, a superconducting current limiting device is constructed using a heat dissipating wafer (e.g., sapphire) having a thin coating of superconducting material deposited onto a surface of the wafer. When a fault is detected, the coating transitions into its normal state and becomes resistive, thereby limiting the flow of current until a circuit breaker, in parallel with the device, interrupts the current flow.
In another conventional approach, a resistive superconducting solenoid is connected within a bridge circuit of diodes or thyristors. In normal operation, current flows through forward-biased pairs of the diodes, bypassing the superconducting solenoid. On the other hand, when the level of current increases above a threshold fault level, the diodes or thyristors become reversed-biased causing the current to flow through the resistive superconducting solenoid which becomes resistive due to the high level of current. In still other approaches, bulk superconducting rods or rings are used to limit the level of fault currents.